Modelling Osmosis

Osmosis can be modelled in the laboratory using Visking tubing (dialysis tubing). This is an artificial, partially permeable membrane with tiny pores. Water molecules can pass through, but larger molecules such as sucrose cannot.A simple osmometer is set up like this:
1. Tie one end of a length of Visking tubing and fill it with concentrated sucrose solution.
2. Fit a narrow capillary tube into the open end and seal it tightly.
3. Clamp the tubing in a beaker of distilled water and mark the starting level of liquid in the capillary tube.Water has a higher water potential outside the tubing than inside, so water enters the tubing by osmosis. The volume inside increases and the liquid level in the capillary tube rises. Measuring the height at regular intervals gives the rate of osmosis.Worked example: the liquid rises 24 mm in 8 minutes. Rate = 24 ÷ 8 = 3 mm min⁻¹.Variables to control include the temperature (which affects the kinetic energy of water molecules), the length and surface area of tubing, and the volume of solution. The independent variable could be the sucrose concentration or the temperature.Limitations of the model: Visking tubing is not living. It has no membrane proteins, so there are no aquaporins, channels or carriers, no active transport and no cell wall. Its pores work by size alone, while a real membrane controls what crosses. The model shows the principle of osmosis, but real cells may behave differently.